1. Technical Field
This invention relates to electrolytic cells. In one aspect, this invention relates to cathode collector bars of electrolytic reduction smelting cells used in the production of aluminum.
2. Background
Aluminum is produced by an electrolytic reduction of alumina in an electrolyte. The aluminum produced commercially by the electrolytic reduction of alumina is referred to as primary aluminum.
Electrolysis involves an electrochemical oxidation-reduction associated with the decomposition of a compound. An electrical current passes between two electrodes and through molten Na.sub.3 AlF.sub.6 cryolite bath containing dissolved alumina. Cryolite electrolyte is composed of a molten Na.sub.3 AlF.sub.6 cryolite bath containing alumina and other materials, e.g., such as fluorspar, dissolved in the electrolyte. A metallic constituent of the compound is reduced together with a correspondent oxidation reaction.
Electrical current is passed between the electrodes from an anode to a cathode to provide electrons at a requisite electromotive force to reduce the metallic constituent which usually is the desired electrolytic product, such as in the electrolytic smelting of aluminum. The electrical energy expended to produce the desired reaction depends on the nature of the compound and the composition of the electrolyte.
Hall-Heroult aluminum reduction cells are operated at low voltages (e.g. 4-5 volts) and high electrical currents (e.g. 70,000-325,000 amps). The high electrical current enters the reduction cell through the anode structure and then passes through the cryolite bath, through a molten aluminum metal pad, and then enters a carbon cathode block. The electrical current is carried out of the cell by multiple cathode collector bars.
As the electrolyte bath is traversed by electric current, alumina is reduced electrolytically to aluminum at the cathode, and carbon is oxidized to carbon dioxide at the anode. The aluminum, thus produced, accumulates at the molten aluminum pad and is tapped off periodically. Commercial aluminum reduction cells are operated by maintaining a minimum depth of liquid aluminum in the cell, the surface of which serves as the actual cathode. The minimum aluminum depth is about 2 inches and may be 20 inches.
The alumina-cryolite bath is maintained on top of the molten aluminum metal pad at a set depth. The current passes through the cryolite bath at a voltage loss directly proportional to the length of the current path, i.e., the interpolar distance gap between the anode and molten aluminum pad. A typical voltage loss is about 1 volt per inch. Any increase of the anode to cathode spacing restricts the maximum power efficiency and limits the efficiency of the electrolytic cell operation.
Much of the voltage drop through an electrolytic cell occurs in the electrolyte and is attributable to electrical resistance of the electrolyte, or electrolytic bath, across the anode-cathode distance. The bath electrical resistance or voltage drop in conventional Hall-Heroult cells for the electrolytic reduction of alumina dissolved in a molten cryolite bath includes a decomposition potential, i.e., energy used in producing aluminum, and an additional voltage attributable to heat energy generated in the inter-electrode spacing by the bath resistance. This latter heat energy makes up 35 to 45 percent of the total voltage drop across the cell, and in comparative measure, as much as twice the voltage drop attributable to decomposition potential.
An adverse result from reducing anode-cathode distance is a significant reduction in current efficiency of the cell when the metal produced by electrolysis at the cathode is oxidized by contact with the anode product. For example, in the electrolysis of alumina dissolved in cryolite, aluminum metal produced at the cathode can be oxidized readily back to alumina or aluminum salt by a close proximity to the anodically produced carbon oxide. A reduction in the anode-cathode separation distance provides more contact between anode product and cathode product and significantly accelerates the reoxidation or "back reaction" of reduced metal, thereby decreasing current efficiency.
The high amperage electrical current passing through the electrolytic cell produces powerful magnetic fields that induce circulation in the molten aluminum pad leading to problems such as reduced electrical efficiency and "back reaction" of the molten aluminum with the electrolyte. The magnetic fields also lead to the unequal depths in the molten aluminum pad and the cryolite bath. The motion of the metal pad increases, sometimes violently stirring the molten pad and generating vortices, and causing localized electrical shorting.
Metal pad depth variations restrict the reduction of the anode to cathode gap and produce a loss in current efficiency. Power is lost to the electrolyte interposed between the anode and cathode blocks. Movement of the molten aluminum metal pad also causes uneven wear on the carbon cathode blocks and may result in early cell failure.
Metal pad turbulence also increases the "back reaction," or reoxidation, of cathodic products, thereby lowering cell efficiency. Metal pad turbulence accelerates distortion and degradation of the cathode bottom liner through attrition and penetration of the cryolite.
Molten aluminum metal pad stirring can be reduced by modifying the bus bar on an existing cell line to reduce the overall magnetic effects.
Whenever the anode-cathode distance is reduced, short circuiting of the anode and cathode must be prevented. In a conventional Hall-Heroult cell using carbon anodes held close to, but separated from, the molten aluminum metal pad, the shorting is caused by an induced displacement of the metal in the pad. Such displacement is caused in large part by the considerable magnetic forces associated with the electrical currents employed in the Hall-Heroult cell electrolysis. For example, magnetic field strengths of 150 gauss can be present in modern Hall-Heroult cells. This metal displacement can take the form of (1) a vertical, static displacement in the pad, resulting in an uneven pad surface such that the pad has a greater depth in the center of the cell by as much as 5 cm; (2) a wave-like change in metal depth, circling the cell with a frequency of 1 cycle/30 seconds; and (3) a metal flow with flow rates of 10-20 cm/second being common. To prevent shorting, the anode-cathode separation must be slightly greater than the peak height of the displaced molten product in the cell. In the case of aluminum production from alumina dissolved in cryolite in a conventional Hall-Heroult cell, such anode-cathode separation is held to a minimum distance, e.g., 4.0-4.5 cm.
Conventional electrolytic reduction smelting cells for the production of aluminum from alumina incorporate a pre-baked carbon anode structure suspended in the molten cryolite bath and an opposite molten aluminum metal pad cathode adjacent the cryolite bath. The molten aluminum metal pad collects on carbon blocks in the bottom of the cell and forms the liquid metal cathode adjacent the cryolite bath. The electrical current is conducted from the anode through the cryolite bath, then through the molten aluminum metal pad, and through the cathode blocks to the external electric bus bar of the cell.
In the conventional cathode today, multiple steel cathode collector bars extend from the external bus bars through each side of the electrolytic cell into the carbon cathode blocks. The steel cathode collector bars are attached to the cathode blocks with cast iron, carbon glue, or rammed carbonaceous paste to facilitate electrical contact between the carbon cathode blocks and the steel cathode collector bars.
The flow of electrical current through the aluminum pad and the carbon cathode follows the path of least resistance. The electrical resistance in a conventional cathode collector bar is proportional to the length of the current path from the point the electric current enters the cathode collector bar to the nearest external bus. The lower resistance of the current path starting at points on the cathode collector bar closer to the external bus causes the flow of current through the molten aluminum pad and carbon cathode blocks to be skewed in that direction. The horizontal components of the flow of electric current interact with the vertical component of the magnetic field, adversely affecting efficient cell operation.
In recognition of the adverse effects that horizontal current components have on cell efficiency, cell designs have been proposed which attempt to reduce the horizontal component of current by changing the basic design of the cathode collector bars. The proposals found in the literature, however, do not account for the practical necessity of preassembling cathode blocks onto the iron collector bars so that the carbon cathode blocks can be reassembled in the bottom of the cell. They also fail to provide designs which are amenable to safe handling by maintenance crews using heavy equipment such as cranes.
One prior aluminum reduction cell attempt to increase cell efficiency by reducing horizontal current components is found in modified connector bars, of a lighter gauge material than the collector bars, which are connected to the collector bars at points distant from the ends of the collector bars. The resistances of the connector bars operate to direct currents drawn from each corresponding collector bar section. The lighter gauge connector bars are weak because of the lighter gauge material used in the connector bars, and they require special conditions to be handled safely by workers and cranes during maintenance operations. Primary smelting facilities for the production of aluminum have hundreds of electrolytic cells with more than two hundred cells connected in series. Because of the large number of cells, cell maintenance is an ongoing operation involving numerous personnel and heavy equipment, such as cranes, to move the heavy carbon cathode blocks and cathode collector bars.
Modified current lead bars positioned perpendicular to the bottom of the electrolytic cell require passages through other portions of cell lining, i.e., through the concrete vault and/or the refractory and insulating brick layers. Such passage would be costly and at the same time create a direct leakage path out of the cell, for any liquid metal or bath that penetrated the cathode block during operation. Such leakage, because of its proximity to the bus, would cause severe damage, thus creating an extended and costly repair prior to the cell being returned to service.
Modified carbon blocks having different resistivities have been arranged such that blocks with higher resistivities are closer to the sides of the cell. This approach requires the use of multiple joints along the length of each composite cathode. These joints are filled or rammed with a carbonaceous paste often referred to as seam mix or ramming paste. The ramming paste is an unfired or green mixture of anthracite and pitch binder, that is rammed into place once the cathode blocks are set in position and then baked to its final consistency immediately prior to the addition of molten bath. Over time, rammed seams have proven to be more susceptible to bath and metal leakage in operation than the pre-baked cathode blocks. Any metal leakage in these block to block joints directly exposes the collector bar to molten metal which results in a shortened pot life. Another concern has been the integrity of the critical cathode block to collector bar joint in the system. Because of the nature of the construction, the cathode to collector bar joint is made by placing the collector bar in the pot, applying a jointing compound to the bar, and then lowering the block into position. Under these conditions, it is extremely difficult to maintain the high quality necessary in this joint and as a consequence, the performance of the pot can suffer.
Prior attempts to solve the current distribution problem in aluminum electrolytic reduction cells fail to provide a practical design which can be implemented without major capital expenditures, provide serviceable pot life, and which is safe to handle by maintenance operators using heavy equipment.